In an MRI apparatus, a pulse sequence is sequentially executed while a phase encoding amount given to a nuclear magnetic resonance signal (hereinafter referred to as “NMR signal”) is varied, and an echo signal necessary for reconstructing one image is thus acquired. Therefore, imaging time depends on time of repetition (TR) and the repetition number of times of pulse sequence. When the rapid imaging is performed, a multi-echo pulse sequence in which plural echo signals can be measured by executing the pulse sequence one time and a pulse sequence in which the TR is shortened to several ms to several ten ms are generally used.
Meanwhile, when a region around a heart, such as a coronary artery, is imaged, it is necessary to perform imaging more rapidly than in the imaging based on the above pulse sequence. To realize such rapid imaging, a rapid imaging method called “parallel imaging method” is proposed.
The parallel imaging method is a method of reducing the repetition number of pulse sequence by executing the pulse sequence using a plurality of receiving coils while phase encoding steps are thinned out at even intervals, and thus shortening the imaging time. When a measured signal obtained by performing the parallel imaging pulse sequence is simply image-reconstructed, an aliasing artifact is generated in the reconstructed image. The aliasing artifact is generated due to a difference between a field of view (FOV) of image to be obtained and that in which signals are actually measured in the parallel imaging, and it can be removed by performing a matrix calculation using a pre-calculated sensitivity distribution of each receiving coil.
Generally, in the parallel imaging method, the imaging time can be shortened depending on the number of receiving coils. That is, it is possible in principle to thin out the number of phase encoding steps matrix as much as the number of receiving coils from the number of phase encoding steps in the usual imaging. As a result, for example, the imaging time can be shortened 1/N times, where the number of receiving coil is N, in comparison with the imaging performed with the usual phase encoding step number.
However, in the parallel imaging method, a sensitivity distribution of each receiving coil is needed for removing the aliasing artifact generated in the image. To obtain the sensitivity distribution of each receiving coil, it is necessary before imaging to execute a pulse sequence for acquiring a sensitivity distribution of receiving coil in the state where the object is put in an imaging space and wearing the receiving coil, in addition to the pulse sequence for obtaining an examination image of the object in which the phase encoding steps are thinned out. Moreover, when imaging is performed on many slices on various positions in an imaging portion of the object to conduct diagnosis, it is necessary to execute the pulse sequence for the sensitivity distribution acquisition on every slice position to be imaged. Therefore, there is a problem that the total imaging time in the parallel imaging becomes very long.
Techniques of shortening the total imaging time in the parallel imaging includes one in which the sensitivity distribution of receiving coil is acquired by performing three-dimensional measurement. However, because the sensitivity distribution data are similarly acquired on every imaging slice position, the effect of shortening the time is small, and it is desired that the time be further shortened.
Furthermore, when the imaging is performed again on a different slice position after measuring the sensitivity distribution of receiving coils in advance of picturing and imaging the object, it is necessary to re-measure the sensitivity distribution of receiving coil on the present imaging slice position.
A first object of the present invention is to provide an MRI apparatus which can shorten the total imaging time when plural slices of the imaging portion of the object are pictured using the parallel imaging method.
A second object of the present invention is to provide an MRI apparatus which can acquire the sensitivity distribution of receiving coil necessary for removing the aliasing artifact within a short time when the object is imaged using the parallel imaging method.
A third object of the present invention is to provide an MRI apparatus which can acquire the sensitivity distribution of receiving coil necessary for removing the aliasing artifact in a simple manner when the object is imaged using the parallel imaging method.